The applicant has previously proposed a variety of forms of tamper resistant enclosures, such as those described in U.S. Pat. No. 5,858,500, and UK Patent Applications 2 220 513 A, 2 258 075 A, 2 256 956 A, 2 256 957 A, 2 256 958 A and 2 270 785 A, the disclosures of which are incorporated herein by reference. These enclosures are in the form of envelopes and shallow boxes, the walls of which are formed by folding flexible sheets incorporating tamper detection features. Included in such sheets are layers of flexible material including a matrix of semi-conductive lines printed on thin insulating film. The matrix of lines forms a continuous conductor which is broken if attempts are made to penetrate the film. The circuit is monitored by opening the conductor at one point and measuring the resistance between the two ends of the circuit. The sheets are folded and overlapped to create an enclosure of wedge-shaped, cuboid or cube form, such as disclosed in GB 2 258 075 A in which a laminate is folded about a plurality of fold lines to form an enclosure. In U.S. Pat. No. 5,858,500 the envelope or box formed of the flexible sheet is potted in a settable material. The enclosures are intended to surround an item to be protected, such as an electronic device, which may be an encryption module, chip or other circuit for processing, containing or carrying potentially valuable information. As noted above, any attempt to penetrate the enclosure results in damage to one or more of the lines, and this damage is detectable as a change in an electrical characteristic of the conductor. On detection of such a change the valuable information contained within the item is typically erased or destroyed and an alarm may be activated.
It is, however, a limitation of such an enclosure system that should access to any components within the enclosure be required e.g. for maintenance or replacement of components, the whole has to be reassembled and wrapped. The discussed tamper respondent enclosures also suffer from the limitation in that the enclosures may only be used once. This means that once the cover has been removed, the tamper respondent sensor has to be completely replaced. This adds to the cost of using such enclosures.
It is an object of at least one embodiment of the present invention to obviate or mitigate at least one or more of the aforementioned limitations.
It is a further object of at least one embodiment of the present invention to provide a tamper respondent enclosure which may be used more than once.